[ThaVirus]

Originally Posted by Frazod

Spoiler!

IIRC there is some sort of horn that can be used to dispel the Wall's power. Can't remember any details, though.

Spoiler!

The Horn of Joramun aka Horn of Winter.

They say, when it's sounded, it wakes giants from the earth or some shit. It probably causes earthquakes.

There are lots of theories out there, even that it doesn't exist/no one has found it. Assuming it is real and has been seen at some point in the series, I'd have to assume it's that broken horn Jon found with the dragon glass near the Fist.